Field of Technology
The present disclosure relates to an austenitic stainless steel. In particular, the disclosure relates to a cost-effective stabilized austenitic stainless steel composition including low nickel and molybdenum levels, improved high temperature properties and at least comparable corrosion resistance and formability properties relative to higher nickel alloys.
Description of the Background of the Technology
Austenitic stainless steels exhibit a combination of highly desirable properties that make them useful for a wide variety of industrial applications. These steels possess a base composition of iron that is balanced by the addition of austenite-promoting and stabilizing elements, such as nickel, manganese, and nitrogen, to allow additions of ferrite-promoting elements, such as chromium and molybdenum, which enhance corrosion resistance, to be made while maintaining an austenitic structure at room temperature. The austenitic structure provides the steel with highly desirable mechanical properties, particularly toughness, ductility, and formability.
A specific example of an austenitic stainless steel is AISI Type 316 stainless steel (UNS S31600), which is a 16-18% chromium, 10-14% nickel, and 2-3% molybdenum-containing alloy. The ranges of alloying ingredients in this alloy are maintained within the ranges specified in order to maintain a stable austenitic structure. As is understood by one skilled in the art, nickel, manganese, copper, and nitrogen contents, for example, contribute to the stability of the austenitic structure. However, the rising costs of nickel and molybdenum have created the need for cost-effective alternatives to S31600 which still exhibit high corrosion resistance and good formability.
Another alloy alternative is Grade 216 (UNS S21600), which is described in U.S. Pat. No. 3,171,738. S21600 contains 17.5-22% chromium, 5-7% nickel, 7.5-9% manganese, and 2-3% molybdenum. Although S21600 is a lower nickel, higher manganese variant of S31600, the strength and corrosion resistance properties of S21600 are much higher than those of S31600. However, as with the duplex alloys, the formability of S21600 is not as good as that of S31600. Also, because S21600 contains the same amount of molybdenum as does S31600, there is no cost savings for molybdenum.
A variant of S31600 also exists which is primarily intended for use at high temperatures. This alloy is designated as Type 316Ti (UNS S31635). The significant difference between S31600 and S31635 is the presence of a small addition of titanium balanced to the amount of carbon and nitrogen present in the steel. The resulting steel, S31635, is less prone to the deleterious formation of chromium carbides at elevated temperatures and during welding, a phenomenon known as sensitization. Such additions can also enhance elevated temperature properties due to the strengthening effects of primary and secondary carbide formation. The specified range for titanium in S31635 is given by the following equation:[5×(% C+% N)]≦Ti≦0.70%However, S31635 uses costly raw material.
Other examples of alloys include numerous stainless steels in which nickel is replaced with manganese to maintain an austenitic structure, such as is practiced with Type 201 steel (UNS S20100) and similar grades. However, there is a need to be able to produce an alloy having a combination of improved elevated temperature properties similar to S31635 and both corrosion resistance and formability properties similar to S31600, while containing a lower amount of nickel and molybdenum so as to be cost-effective. In particular, there is a need for such an alloy to have, unlike duplex alloys, a temperature application range comparable to that of standard austenitic stainless steels, for example from cryogenic temperatures up to 1300° F.
Accordingly, the present invention provides a solution that is not currently available in the marketplace, which is a formable stabilized austenitic stainless steel alloy composition that has comparable corrosion resistance properties and improved elevated temperature properties to S31600 and S31635, while providing raw material cost savings. Accordingly, the invention is a stabilized austenitic alloy that uses controlled levels of carbide-forming elements to improve elevated temperature properties. The austenitic alloy also utilizes a combination of the elements Mn, Cu, and N, to replace Ni and Mo in a manner to create an alloy with similar properties to those of higher nickel and molybdenum alloys at a significantly lower raw material cost. Optionally, the elements W and Co may be used independently or in combination to replace the elements Mo and Ni, respectively.